ELECTROMAGNETIC EFFECT OF CONVECTION-CURRENTS 259 



n ]u-t 



But =, = 0-03 nearly, and -^ is approximately unity. . 



. I== (HM-H'_M^^ 



8nM(C + Z>) 

 or 



-f '- =. - 1 1 3 (say) . 



M tan o 



Similarly, for the revolving disks, 



= /? tan J. 



8 , ^ ^ 



^_ O'<- T^~ ' - < 



F /?. J 



For the sine-galvanometer: 



TT 



I = sin <p. 



/. 7=10-* 5-46 ZTsin f, 

 and 



/5 = 10-*. 5-46 



tan P 



For measurement of H : 

 Electrodynamometer, 



ls =0- z jr V sin a. 



^ = constant of windings = 10~ 3 . 6'454. 

 K- moment of inertia = 10 2 . 8-266. 

 T= time of one swing =2-441. 

 .-. i = 10~ 2 . 7-59 Vsin . 



Tangent galvanometer: 



i = |C tan d = ^ tan 8 . 

 2-w 



n = no. turns = 10. 

 b = radius turns = 49-98. 

 .-. t = 0-795 JJ tan d, 



and, substituting the value of t, 



JI=10-'. 9-55 ***. 

 tan d 



